The choice of the mode of prediction of the macroblocks belonging to the images of B type or B images, as per the MPEG 4 standard, is highly dependent on the mode of coding adopted for the “co-located” macroblocks of the last coded predicted reference P image. The MPEG 4 standard ISO/IEC 14496-2 relating to video coding specifies, as far as motion compensation for skipped macroblocks (paragraph 7.6.9.6) is concerned, that if the co-located macroblock which is situated in the I or P video object plane (I-VOP or P-VOP) most recently decoded is skipped, the macroblock belonging to the B images is processed in the forward predictive mode (“forward prediction”) with the zero motion vector.
Stated otherwise, if a macroblock of the P image is skipped, that is to say not coded, the “co-located” macroblocks belonging to the previous images of B type, in the order of display, have necessarily to be reconstructed by a specific mode of interpolation utilizing an earlier, in the order of display, reference image via zero vectors.
This concept has been worked out starting from the principle that the content of an image sequence complies with a minimum of temporal stationarity between two images of I or P types. This is true for contents which may be referred to as conventional (TV scene, cinema, videophone, videoconference). On the other hand, as soon as one is confronted with montages, video clips, footage of films endowed with special effects, this stationarity 4 is no longer complied with and an image may not bear any relation to the content of the previous image and next image. If it turns out that this isolated image is of B type, the consequence is that the macroblocks of this B image, which have their counterparts “skipped” in the last coded P image, are inevitably reconstructed with information unrelated to the initial information. Visually the result, albeit momentary, is catastrophic and is characterized by defects in the size of the macroblock.
FIG. 1 illustrates the problem which may arise with uncoded macroblocks in a P-VOP (image of P type). An image sequence consists for example of four successive images VOP 1 to VOP 4. The images VOP 1 and VOP 4 are P type images (P-VOP), the images VOP 2 and VOP 3 are B type images (B-VOP). The P-VOP most recently decoded is VOP 4. When B-VOP 3 is decoded, some of the macroblocks of P-VOP 1 are copied over as they are into B-VOP 3, insofar as the co-located macroblocks of P-VOP 4 have not been coded, these macroblocks of P-VOP 4 then being likewise a copy over of the macroblocks of P-VOP 1. Thus, a number of black macroblocks are thus seen to appear in this B-VOP.
The aim of the invention is to alleviate the aforesaid drawbacks.